Current generation and control systems for electrolytic vat

ABSTRACT

A current generation and control system for electrolytic processes in an electrolytic vat in which two autotransformers are provided each having a primary part coupled to a regulator for adjusting the number of coils and driven by control from a microprocessor and a secondary part coupled to a respective input terminal of the electrolytic vat and to a respective half-wave rectifier also controlled by the microprocessor wherein the two rectifiers including thyristors are coupled between the autotransformers and the respective input terminals in counter position to suppress the negative and positive half-waves of the voltages generated by the autotransformers.

OBJECT OF THE INVENTION

The present invention relates to a number of improvements to currentcontrol systems used in electrolytic processes such as the conventionalelectrolytic coloration processes, opacification processes, processesfor obtaining a range of greys, and aluminum optical interferencecoloration processes, though clearly such improvements can also beapplied to any other field requiring like current control systems.

BACKGROUND OF THE INVENTION

For aluminium electrolytic coloration processes to be carried out tofull satisfaction, a very thorough control on the current applied mustexist.

Thus, for instance, Spanish patent of invention no. 498,578 and its U.S.Pat. No. 4,421,610, sets forth an electrolytic coloration process for analuminium or aluminium alloy element, consisting of a first phase where,inter alia, an alternating current with a peak voltage lying between 25and 85 volts and a current density below 0.3 amps. per square decimetermust be applied.

More specifically, and in order to obtain such alternating current, apolyphasic network or the secondaries in a polyphasic networktransformer are used conducting the positive and negative half-cycleswith the same conduction angle and both variables as required, whichconduction angles are in turn controlled by reverse shunt thyristors orby triacs.

Said control of the thyristors' conduction angle obviously allows theaverage voltage to be controlled, but not so the peak voltage, andtherefore the results attained, though acceptable, cannot be deemed tobe the most favourable.

Manifold solutions have been put forward so far as electrolyticcoloration processes are concerned, and the essential problem common toall is the difficulty of suitably controlling the currents applied tothe vat.

Furthermore, from the theoretical viewpoint , opacification processesare known to attain, likewise by electrolytic processes, atransformation of the anodic film rendering the same opaque, but suchprocesses require very low voltages in practice, less than three volts,and moreover very specific values, and no current control means existspresently that may allow the same to be maintained within the limits theprocess requires.

Optical interference aluminum coloration processes are also known, wherethe above-mentioned problem is even worse, for within a given range ofvoltages, minor variations in the value of the voltage lead tosignificant changes in the colour obtained, for which reason this systemhas not been developed industrially either, for the different loadcharacteristics and the actual installation determine variations in thevoltage drop and, hence, variations in the voltage applied to the load,originating undesirable colour changes.

There is hence no doubt whatsoever that the fact that there arepresently no suitable means for controlling the current applied toelectrolytic processes significantly constrains progress in this field.

In order to grasp the difficulties of the different aluminumelectrolytic coloration systems it is worthwhile to note some of thephenomena that take place when applying an alternating current to thepreviously anodized aluminum:

During the positive half-cycle there is no deposition whatsoever at theanodic film pores. In the event of the voltage applied allowing passageof current, oxidation takes place, leading to an increase in filmbarrier thickness. The final film barrier thickness is proportional tothe peak voltage applied.

During the negative half-cycle there is a double deposition. On theother hand, deposition of the metallic cation present in the form of ametallic particle. For instance:

    Sn.sup.2 +2e.sup.- --Sn

Furthermore, deposition of protons present in the electrolyte, thatbecome atomic hydrogen:

    H.sup.+ +1e.sup.- --H

The speed of migration of the protons toward the bottom of the poresdepends upon the voltage applied and the density of the circulatingcurrent. This latter in turn depends upon the total circuit impedance(see electric model of the U.S. Pat. No. 4,421,602, namely FIG. 1thereof).

Because of the semiconducting nature of the film barrier, atomichydrogen can be formed at low voltages, for instance at roughly 2 to 4V. As higher voltages are applied and current circulation rises, thishydrogen can act differently:

    GH+Al.sub.2 O.sub.3 --2Al.sup.3+ +3H.sub.2 O               a)

    H+Sn.sup.2+ --Sn+2H.sup.+                                  b)

    H+H--H.sub.2                                               c)

Reaction a) takes place at voltages under 7-8 V.

Reactions b) and c) take place at voltages in excess of 8 V.

When the kinetic energy of the protons is very high, or film barrierresistance is weak, the protons can cross the film barrier and reactionc) can take place at the metal-oxide interface. In such event, thepressure generated by the accumulation of the molecular hydrogen formedcan cause spalling.

These three types of effects caused by hydrogen can be regulated byaccurately controlling the voltage applied during the negativehalf-cycle. The voltage in the positive half-cycle must be adjustedsimultaneously to keep the circuit's impedance under control.

Thus:

With a), the bottom of the pores can be modified to cause the filmbarrier to become opaque, or the film barrier diameter and thicknessadjusted in order to subsequently obtain the optical interferencecolours.

With b), the formation of metallic particles at the bottom of the porescan be enhanced; cations, for instance Sn²⁺.

Effect c) can be regulated by the separate positive half-cycle voltagecontrol, that allows film barrier thickness to be increased, thereby toincrease resistance and prevent spalling.

By analyzing these three effects, it can be clearly inferred that it isnecessary to regulate and control the positive and negative half-cyclevoltages and currents separately.

In electrolytic coloration processes, the passage of current is usuallycontrolled and regulated indirectly by adjusting and controlling thevoltage applied to the electric circuit (see FIG. 1 in U.S. Pat. No.4,421,610). This adjustment is made through programs that linearlymodify the voltage according to time.

The voltage must be modified as circuit impedance changes. If circuitimpedance variation is not linear, neither can voltage variation be so.Thus, certain mathematical algorithms similar to those relating circuitimpedance variations during the process must be applied at the voltageadjustment programs.

DESCRIPTION OF THE INVENTION

The improvements to the current control systems subject hereof fullysolve the aforesaid problems, allowing the voltage applied to beaccurately adjusted at all times to meet requirements under thetheoretical process being put in practice.

More specifically, and in order to achieve the above, such improvementscomprise two shunted autotransformers, each such autotransformer beingprovided with a duly controlled half-wave rectifier, thereby to take thepositive half-wave of the resulting voltage from one of theautotransformers, and the negative half-wave from the otherautotransformer.

Both these autotransformers, theoretically in step, may in practiceundergo phase displacement leading to short circuit problems, to whichend it has been foreseen, as another characteristics of the invention,that the conduction angle of the thyristors provided in the aforesaidrectifiers be cut for safety, specifically affecting the positive and/ornegative half-waves near the phase reversal area, where those shortcircuit problems deriving from a possible displacement of either phasecan originate.

To supplement the said structure, and as yet another characteristic ofthe invention, the current control system is provided with amicroprocessor, carrying, as appropriate, an operative program suitablefor the process to be carried out by mathematical algorithms, whichmicroprocessor will "read" the voltage being applied to the load at alltimes through sensors duly established at the input to the vat, andthat, when the latter moves away from the established pattern, shall actupon the control means of the autotransformers and the half-waverectifiers, to achieve the pertinent modifications in such elements inorder to achieve an almost exact precision in the voltage or currentapplied to the load.

DESCRIPTION OF THE DRAWINGS

In order to provide a fuller description and contribute to the completeunderstanding of the characteristics of this invention, a set ofdrawings is attached to the specification which, while purelyillustrative and not fully comprehensive, shows the following:

FIG. 1. Is a diagram showing the current control system for electrolyticprocesses, with the improvements subject hereof.

FIG. 2. Is a voltage time diagram for one of the systemautotransformers, showing possible voltage value variations.

FIG. 3. Is the same diagram as in FIG. 2, but for the secondautotransformer.

FIG. 4. Is the voltage diagram for the first autotransformer afterpassage through the first half-wave rectifier.

FIG. 5. Is the same diagram as in FIG. 4, but for the secondautotransformer.

FIG. 6. Is the same diagram as in the previous figures, but showing theinput to the vat, i.e., the summation of both autotransformers.

FIG. 7. Is the same diagram as in the previous figure, but with a phasedifference between both autotransformers that is possible in practice.

FIG. 8. Is the same diagram as in FIG. 7, with the phase difference inthe opposite direction to that of the said figure.

FIG. 9. Is the voltage diagram of FIG. 6 after providing the thyristors'conduction angle with a suitable cut in order to avoid the problemsshown in the diagrams of FIGS. 7 and 8.

FIG. 10. Is, based upon the voltage waves cut in the previous figure,the phase difference between both autotransformers and the absence ofshort circuit effects.

FIG. 11. Is a voltage/time diagram of an embodiment of the electrolyticcoloration system.

FIG. 12. Is a voltage/time diagram of an embodiment of the opacificationsystem.

FIG. 13. Is the same diagram as in FIGS. 11 and 12, but for greyelectrolytic coloration.

FIG. 14. Is the same diagram as in FIGS. 1 through 13, but for anoptical interference pre-coloration phase.

FIG. 15. Is, finally, another voltage/time diagram, in this case forblue coloration.

PREFERRED EMBODIMENT OF THE INVENTION

In light of the above figures, and more specifically FIG. 1, it can beobserved that the improvements to the current control systems subject ofthe invention comprise the use of two autotransformers (1) and (2)shunted to a given phase (3) of the mains, the primary of suchautotransformers being provided with a regulator (4), of anyconventional sort, driven automatically to allow the number of coilsthat are effective from the viewpoint of transformation to be varied,while the secondary of such transformers (1) and (2) is fitted with twohalf-wave rectifiers (5) and (6) situated in counterposition, so thatwhile the rectifier (5) suppresses the negative half-wave of the currentgenerated by the autotransformer (1), the rectifier (6) suppresses thepositive half-wave of the current generated by the autotransformer (2),such autotransformers being, as aforesaid and beyond the half-waverectifiers, shunted to the terminals (7) representing the input orconnection to the electrolytic vat (8), one of the terminals beingconnected to the load (9) and the other to a counterelectrode (10).

A microprocessor (11) permanently controls the voltage at the input (7)to the vat (8) through the connection (12) detecting contingent driftsof such voltage or current in either direction with regard to thetheoretical value foreseen, so that, with a suitable program, using themathematical algorithms, it shall act on the autotransformers' (1) and(2) regulators (4), and on the rectifiers (5) and (6), to reset suchtheoretical and hence most ideal value.

According to this structure and as aforesaid, a symmetric sine wave ofvariable value as shown in FIG. 2 will be obtained at theautotransformer (1) output, adjustable at will through the saidregulator (4), as is the case of the autotransformer (2), that willprovide an output symmetric sine wave signal as shown in FIG. 3.

The half-wave rectifier (5) will suppress the negative half-waves fromthe autotransformer (1) output, as shown in FIG. 4, whilst the half-waverectifier (6) will do the same at the autotransformer (2) output withthe positive sine waves, as shown in FIG. 5. As both autotransformersare shunt-fed, an asymmetric sine wave will appear at their commonoutput (7), as shown in FIG. 6, the summation of the voltages that arein turn shown in FIGS. 4 and 5.

In practice and because of problems that have nothing to do with theactual electrolytic installation, there will be phase differencesbetween the voltages generated by both autotransformers, in thedirection shown in FIG. 7 or in the opposite direction shown in FIG. 8,and to such end, acting on the thyristors provided in the half-waverectifiers (5) and (6), both the positive and the negative half-wavesare provided with a slight cut at their areas closest to the zero valuepoints for voltage, as shown in FIG. 9, and therefore in the event of aphase difference as aforesaid, such cuts prevent the overlap of voltagesin the opposite direction, as is in turn shown in FIG. 10, and theresulting short circuits that would derive from such partial overlaps.

EXAMPLES Example 1

Bronze electrolytic coloration.

Anodizing phase: The element to be treated was previously anodized in abath comprising sulphuric acid at a concentration of 180 g/l, at atemperature of 20° C., and under a current density of 1.5 A/dm² for 35minutes.

Coloration phase: The anodized element underwent electrolytic colorationin a bath comprising:

    ______________________________________                                        SO.sub.4 Ni.7H.sub.2 O                                                                           35 g/l                                                     SO.sub.4 Sn        10 g/l                                                     O-phenol sulphonic acid                                                                           2 g/l                                                     SO.sub.4 H.sub.2   15 g/l                                                     ______________________________________                                    

and an asymmetric alternating voltage as shown in FIG. 11 was applied.Such figure shows the voltage variations of half-cycles A and Bseparately.

The following colours were obtained in the following times:

    ______________________________________                                               Light Bronze                                                                            1'                                                                  Medium Bronze                                                                           2'                                                                  Dark Bronze                                                                             3'                                                                  Black Bronze                                                                            10'                                                          ______________________________________                                    

Example 2

Grey electrolytic coloration.

Anodizing phase: The element to be treated was previously anodized in abath comprising:

    ______________________________________                                        SO.sub.4 H.sub.2                                                                              180 g/l                                                       Glycerine       3 g/l                                                         Oxalic acid     5 g/l                                                         Ethylene glycol 1 g/l                                                         ______________________________________                                    

under the following conditions:

    ______________________________________                                        current density      1.7 A/dm.sup.2                                           temperature          20° C.                                            time                 40 minutes                                               ______________________________________                                    

Opacifying phase: The anodized element was treated in a bath comprising:

    ______________________________________                                               SO.sub.4 H.sub.2                                                                      150 g/l                                                               Oxalic acid                                                                           20 g/l                                                                Glycerine                                                                              3 g/l                                                                Al.sup.3+                                                                             25 g/l                                                         ______________________________________                                    

at a temperature of 20° C.

A symmetric alternating voltage as shown in FIG. 12 was applied. Suchfigure shows the voltage variations of half-cycles A and B separately.

After ten minutes a uniform opaque-whitish film was obtained.

Coloration phase: The opacified element underwent electrolyticcoloration in a bath comprising:

    ______________________________________                                        SO.sub.4 Ni.7H.sub.2 O                                                                           35 g/l                                                     SO.sub.4 Sn        10 g/l                                                     O-phenol sulphonic acid                                                                           2 g/l                                                     SO.sub.4 H.sub.2   15 g/l                                                     ______________________________________                                    

and a symmetric alternating voltage as in FIG. 13 was applied. Suchfigure shows the voltage variations of half-cycles A and B separately.The following colours were obtained in the following times:

    ______________________________________                                               Light Grey                                                                              30"                                                                 Medium Grey                                                                             1'                                                                  Dark Grey 2'                                                                  Black Grey                                                                              5'                                                           ______________________________________                                    

Example 3

Blue optical interference coloration.

Anodizing phase: The element to be treated was previously anodized in abath comprising:

    ______________________________________                                        SO.sub.4 H.sub.2                                                                              180 g/l                                                       Glycerine       3 g/l                                                         Oxalic acid     5 g/l                                                         Ethylene glycol 1 g/l                                                         ______________________________________                                    

under the following conditions:

    ______________________________________                                        current density      1.7 A/dm.sup.2                                           temperature          20° C.                                            time                 40 minutes                                               ______________________________________                                    

Precoloration phase: The anodized element was treated in a bathcomprising:

    ______________________________________                                               SO.sub.4 H.sub.2                                                                      150 g/l                                                               Oxalic acid                                                                           20 g/l                                                                Glycerine                                                                              3 g/l                                                                Al.sup.3+                                                                             25 g/l                                                         ______________________________________                                    

at a temperature of 20° C.

An asymmetric alternating voltage as shown in FIG. 14 was applied. Suchfigure shows the voltage variations of half-cycles A and B separately.

After six minutes the process was stopped.

Coloration phase: The element, after having gone through theprecoloration treatment, underwent coloration in a bath comprising:

    ______________________________________                                        SO.sub.4 Ni.7H.sub.2 O                                                                      35 g/l                                                          SO.sub.4 (NH.sub.4).sub.2                                                                   20 g/l                                                          BO.sub.3 H.sub.3                                                                            30 g/l                                                          SO.sub.4 Mg    5 g/l                                                          SO.sub.4 H.sub.2                                                                            up to pH 4.2-4.7                                                ______________________________________                                    

An asymmetric alternating voltage as in FIG. 15 was applied. Such figureshows the voltage variations of half-cycles A and B separately.

After two minutes of this treatment, a deep blue colour was obtained.

We feel that the device has now been sufficiently described for anyexpert in the art to have grasped the full scope of the invention andthe advantages it offers.

The materials, shape, size and layout of the elements may be alteredprovided that this entails no modification of the essential features ofthe invention.

The terms used to describe the invention herein should be taken to havea broad rather than a restrictive meaning.

I claim:
 1. A current generation and control system for electrolyticprocesses in an electrolytic vat having a load and a counterloadtherein, the system comprising two autotransformers each having aprimary part and a secondary part and being shunted to the same phase;each autotransformer including an automatically driven regulator coupledto said primary part thereof for automatically controlling the number ofcoils being operative at all times, said electrolytic vat having twoinputs of which one input is coupled to said load and another input iscoupled to said counterload, the secondary part of one of saidautotransformers being coupled to said one input and the secondary partof another of said autotransformers being coupled to said another input;two half-wave rectifiers each coupled between the respective input ofthe electrolytic vat and the secondary part of the respectiveautotransformer such that said rectifiers act on opposite half-waves sothat while one rectifier suppresses a negative half-wave from a voltagegenerated by one autotransformer another rectifier suppresses a positivehalf-wave of the voltage generated by another autotransformer to yield asine wave voltage with symmetric or asymmetric positive and negativehalf-waves at said inputs; and a microprocessor coupled to saidregulators so as to control an output voltage of said autotransformers,and to said rectifiers so as to control said positive and negativehalf-waves separately, each rectifier including a thyristor.
 2. Thecurrent generation and control system according to claim 1, wherein saidmicroprocessor is further coupled to said inputs for detectingcontingent drifts of the sine wave voltage in either direction forresetting said transformer accordingly and wherein thyristors of saidrectifiers control output voltages of said autotransformers to avoidshort circuit problems which may be caused by an overlap of half-wavesin opposite directions due to possible phase shifts.